vi DETERV N OF ROCKS 69 



may be distinguished '-urc. and 



combustion. Clays or shale*, rendered black by the 

 vegetable matt < onuun, may be recognised by 



reak, and their turning white but retu 

 their shape when strongly he.. : blai k heavy rocks 



abound in whu h tin rr : - n. trace of carbon. These 

 very generally contain a considerable amount of iron, 

 either in the form of magnetite, ilmenite, or other related 

 oxide, or in that of some black ferruginous mineral, such 

 as hombK .. h rocks are apt to weather with a 



brown or yellow crust, owing to the conversion of the 



the hydrous peroxide. 



Rroum. This colour characterises some rocks on : 

 fresh fractures, as the v ^tone called black- 



band. A few crystalline rocks have a brown tint from 

 the presence of minerals of that colour, such as varieties 

 *-a and garnet Hut it is more jurticularly on the 

 decomposed surfaces and crusts of rocks that brown tints 

 appear. The iron is there convened into the hydrous 

 peroxide (hydrated ferric oxide, limonite). Basalt-rocks 

 show this change in a most in>tnu tive manner. Earthy 

 manganese also gives dark brown to black tints. 



/<*. The colouring material of yellow rocks is 

 almost always limonite. Yellow sandstones, beds of 

 ochre, the weathered crusts of many limestones and of 

 numerous ferruginous crystalline rocks furnish illustra- 

 tions. Sometimes a metallic or brassy yellow is com- 

 municated to parts of rocks by diffused in> 



this yellow is of the pale kind due to marca 

 can only be seen on fresh fractures, as it disappear 

 the rapid decomposition of the mineral 



